This invention relates to photo cathodes in general and more particularly to an improved method of manufacturing a photo cathode for electroradiographic and electrofluoroscopic apparatus.
U.S. application Ser. No. 889,524 filed Mar. 23, 1978 and assigned to the same assignee as the present invention describes a photo cathode for electroradiographic and electrofluoroscopic apparatus which contains a stacked arrangement of perforated foils of a material with a high atomic number. The perforated foils of this photo cathode can advantageously be made as perforated double layer films with two outer, electrically conductive layers and an insulating layer disposed in between, a predetermined potential gradient being provided between the two outer layers.
Similar photo cathodes can be provided, especially for apparatus in the so-called low pressure ionography in medical technology (Phys. Med. Biol. 18 (1973), pages 695 to 703). In such equipment, the external X-ray photo effect of a solid-state photo cathode is used for generating electric charge carriers. The emitted photo electrons are subsequently multiplied in the gas space of a corresponding chamber by means of a Townsend discharge to such an extent that an electrostatic image that can be developed is produced on a paper or plastic foil. If an electroluminescent screen is used for collecting the charges instead of these foils, a process changing in time can also be displayed with this method in image sequences. Such a method is called electrofluoroscopy. A well known embodiment example of this is the X-ray image amplifier.
If a suitable filling gas, which may be at atmospheric pressure, is used in the chamber of such a photo cathode, amplification factors of 10.sup.4 can be obtained without difficulty. However, there is a great discrepancy between the depth of penetration of the X-rays and the range of the emitted photo eletrons. Due to this discrepancy, which is around 100:1, special measures must be taken for the photo cathodes to attain a quantum yield which will meet the requirements of medical technology regarding sensitivity and resolution. Quantum yield is understood here to be the number of photo electrons emitted per incident X-ray quantum. With the photo cathode mentioned at the outset with a stacked arrangement of perforated foils of a material with a high atomic number, relatively high absorption of the X-rays and thereby, correspondingly high quantum yield is now possible, since the quantum yield is essentially the product of the photo absorption coefficient and the range of the electrons and depends on the energy of the radiation and the atomic number of the cathode material. In addition, the quantum yield of the photo cathode mentioned at the outset is substantially higher than the quantum yield of a comparable solid, plane photo cathode because of the larger effective surface area due to the stacked arrangement of the perforated foils. The electron emission capacity of such a cathode increases proportionally to the larger surface as long as an attenuation of the X-rays in these structures is still of secondary importance.
The perforated double layer foils of such a photo cathode can be manufactured, according to Ser. No. 889,524, by first providing the webs on a simple perforated foil with an insulating layer on one side and finally depositing an electrically conductive material on the parts of the insulating layer which cover up the webs. The insulating layers must be as free as possible of disturbances which could lead to a reduction of the dielectric strength of the insulating layer. In the proposed procedure, the effort to achieve this is relatively great.
It is therefore an object of the present invention to describe another method by which perforated double layer foils for a photo cathode of the type mentioned at the outset can be manufactured in a relatively simple manner.